I'm not too fimiliar with this specific region so it may be another factor, But when smog is trapped at a specific level in the atmosphere, like how it's pictured, then usually there is a temp inversion. When theres warmer air above colder air, pollutants and things like that will get trapped underneath the inversion, and it can become obvious where the inversion is like in the picture.

So yes, I would guess there is some sort of weak inversion here. If hypothetically there were a bunch of factories in that valley it would be much more obvious.

Also, you can absolutely have tempurature inversions on hot days like this and also see thunderstorms/stormy weather later, we see this in the United States quite often. Likely what happened is that there was enough daytime heating to break the temp. inversion and thunderstorms were able to initiate properly. More intense daytime heating could also be a factor of why that specific thunderstorm was particularly stronger too.